Array based sensing has emerged as a powerful tool for the detection of chemically diverse analytes. These systems mimic the mammalian gustatory and olfactory systems by producing specificity, not from any single sensor, but as a unique composite response for each analyte. Such cross-reactive sensor arrays have application both as electronic nose technology for the detection of volatiles and gases [1-5], and as electronic tongue technology for the detection of aqueous analytes [6-7].
Conventional sensor arrays typically have been based on a variety of changes in the properties of individual sensors. For example, absorption of the analyte into conductive polymers or polymer composites can change the electrical properties of the polymers or composites. In another example, adsorption of the analyte onto surfaces such as metal oxide surfaces can provide for combustion reactions, oxidation reactions or other electrochemical processes, which can be electrically detected. In yet another example, a single fluorophore can be included in an array of different adsorbent polymers, and the change in composite fluorescence of the array can be measured.
Using a different approach from these conventional sensor arrays, colorimetric sensor arrays are based on optoelectronics. A colorimetric sensor is a sensor that includes one or more materials that undergo a change in spectral properties upon exposure to an appropriate change in the environment of the sensor. The change in spectral properties may include a change in the absorbance, fluorescence and/or phosphorescence of electromagnetic radiation, including ultraviolet, visible and/or infrared radiation.
Colorimetric sensor arrays typically include an array of cross-reactive chemoresponsive dyes, where the colors of the chemoresponsive dyes are affected by a wide range of analyte-dye interactions. Colorimetric arrays have been used for the identification and quantification of a wide range of analytes, both in the gas phase and in aqueous solutions [8-17]. The arrays typically are made simply by printing the hydrophobic chemoresponsive dyes onto a hydrophobic membrane.
The chemoresponsive dyes used in colorimetric sensor arrays typically have been limited to soluble molecular dyes, which are present in a porous film [18-23]. Insoluble, nonporous pigments have not provided sufficient contact between the analyte and the chromophores of the pigment, since the chromophores at the surface of the pigment are a small fraction of the total number of chromophores. Likewise, nonporous films have not provided sufficient contact between a dye in the film and the analyte in the sample.
There are a variety of drawbacks to the use of soluble molecular dyes in porous films for colorimetric sensor arrays. Aggressive solvents, such as halocarbons or aromatics, are typically used for printing the dyes. The dyes can leach into analyte solutions from the porous film. The dyes may be unstable, leading to a limited shelf-life. Crystallization of the dyes after printing on the membrane can render the dyes inactive.
One approach to addressing these drawbacks has been to immobilize the chemoresponsive dyes in sol-gel matrices [24-29]. These sol-gel matrices, however, have had poor adherence to the hydrophobic surfaces used for sensor arrays. Thus, the sol-gel based dyes typically have been prepared as a film or a monolithic disk, and have been used individually rather than as an array with other chemoresponsive dyes.
It would be desirable to provide a colorimetric sensor array having increased stability relative to conventional colorimetric arrays, and that does not undergo leaching of soluble dyes during use. Ideally, such a sensor array would include a variety of different chemoresponsive colorants, including dyes and pigments. It would also be desirable for such an array to be formed by a method that does not include aggressive solvents, and that is compatible with reproducible, high-throughput fabrication.